Infrared radiation is that portion of the electromagnetic spectrum between visible light (0.72 microns (.mu.)) and microwave (1000.mu.). The infrared region is subdivided into near infrared (0.72.mu.-1.5.mu.), middle infrared (1.5.mu.-5.6.mu.), and far infrared (5.6.mu.-1000.mu.).
When an object passes in close proximity to an infrared source, infrared energy penetrates the material of that object and is absorbed by its molecules. The natural frequency of the molecules is increased, generating heat within the material, and the object becomes warm. Every material, depending upon its color and atomic structure, absorbs certain wavelengths of infrared radiation more readily than other wavelengths. Middle infrared is more readily absorbed by a greater number of materials than is the shorter wavelength near infrared radiation.
One type of infrared source is the "focused" emitter. This type emits a specific wavelength of infrared energy--usually in the near infrared region--which is a wavelength easily reflected and not readily absorbed by many materials. To compensate for this lack of penetration the intensity of such emitters is increased and reflectors are used to focus the emission on the process area. Increased intensity causes increased power consumption, hotter emitter operation requiring cooling systems, shorter emitter life, and damage to temperature-sensitive product loads which are being heated. Further, the condensation of process vapors on the reflector and emitter surfaces may cause a loss of intensity. Focused infrared sources generally require a substantial energy input, convert only 20 to 59% of the input energy to infrared radiation, and have a life expectancy of approximately 300 hours.
A well-known focused emitter is the T-3 lamp which consists of a sealed tubular quartz envelope enclosing a helically-wound tungsten filament (resistive element) supported by small tantalum discs. The tube is filled with an inert gas such as a halogen or argon to reduce oxidative degeneration of the filament. Due to the different thermal expansion coefficients of the quartz and the metal lead wires adequate cooling must be maintained at the seals or lamp failure will result. The T-3 lamp, when at rated voltage, operates at a peak wavelength of 1.15.mu. with a corresponding filament temperature of 2246.degree. C.
Another commonly used focused emitter is the Ni/Cr alloy quartz tube lamp which is similar to the T-3 lamp in construction except that the filament is contained in a non-evacuated quartz tube. This infrared source, when at rated voltage, operates at a peak wavelength of 2.11.mu. with a corresponding filament temperature of 1100.degree. C.
Nonfocused infrared panel emitters are available which operate on the secondary emission principle. Panel emitters contain resistive elements which disperse their energy to surrounding materials which in turn radiate the infrared energy more uniformly over the entire process area and across a wider spectrum of colors and atomic structures.
The resistive element of such panel emitters is typically a coiled wire or crimped ribbon foil and is placed in continuous channels which extend back and forth across the area of the panel. The curved portions of the channels at each end of the panel area limit the proximity of the wire or foil in adjacent channels. As a result, this construction limits the coverage of the panel area by the resistive element to 65 to 70% and this limited coverage makes it difficult to obtain precise temperature uniformity across the panel emitting surface.
Another known panel emitter comprises a glass emitting layer coated with tin oxide which serves as the resistive element. The tin oxide layer is applied by an expensive vapor deposition process.